<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        .c1{
            width: 100%;
            height: 5000px;
            background-color: gray;
        }

        .c2{
            width: 80px;
            height: 40px;
            background-color: #222222;
            color: white;
            /*里面文本的位置*/
            text-align: center;
            line-height: 40px;
            /*固定定位*/
            position: fixed;
            right: 20px;
            bottom: 20px;
        }
    </style>
</head>
<body>

<div class="c1"></div>

<!--固定定位，定位在右下角，会盖住下面一层的文字-->
<div class="c2">返回顶部</div>

</body>
</html>